Previously, 8 rhesus monkeys (RM) were inoculated with SIV + M. leprae (ML) together with 4 control RM which received only ML. Later, 5 RM which had been inoculated with ML more than 2 years earlier were inoculated with SIV. Last year, these studies were enlarged by beginning a study in which 18 RM were divided into 3 groups of 6. One group was inoculated with SIV and, after 2 weeks, with ML. Another group was inoculated with SIV + ML at the same time and the third group was given ML alone. The results of studies with the 8 experimental and 4 control RM were consistent with our earlier results that indicated increased susceptibility to leprosy in the presence of SIV. Last year, one of the 5 RM that received SIV more than two years after ML developed lepromatous leprosy and was humanely sacrificed. The remaining 4 RM in that group are still being observed. The 18 RM in the most recent study are under observation, but it is as yet too soon to have sufficient clinical results.